This work was supported by the U.S. DOE Office of Science under Grant No. DEFG02-88ER Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016 Indiana University: T. Steinbach, J. Vadas, V. Singh, B. Wiggins, J. Huston, S. Hudan, RdS Florida State: I. Wiedenhover, L. Baby, S. Kuvin, V. Tripathi Vanderbilt University: S. Umar, V. Oberacker Forging elements in a flash Only elements Z=1-4 produced in the Big Bang Fundamentals of supernova explosions are not understood! Synthesis of the heavy elements is not understood Limits of nuclear stability (superheavy elements, N/Z exotic) poorly known Chemical diversity The Chart of the Nuclides and Terra Incognita Radioactive beam facilities allow one to produce neutron-rich nuclei along the r-process path. Neutron stars represent an extreme point on this diagram Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016 The two nuclei have to overcome the Coulomb repulsion (barrier) in order to experience the attraction of the strong nuclear force which is short range. Overcoming barriers Particle can tunnel (exist in classically forbidden region) and emerge on other side. SENSITIVE TO WIDTH OF BARRIER Effect of barrier is observed even if E > U Measuring the fusion cross-section is therefore intrinsically related to measuring the detailed shape of the fusion barrier (a dynamic quantity as the nuclei approach) (Z 1,A 1 ) (Z 2,A 2 ) (Z 1 +Z 2, A 1 +A 2 ) Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016 Fusion reactions in the outer crust are responsible for the X-ray bursts and superbursts Problem: At the temperature of the crust, the Coulomb barrier is too high for thermonuclear fusion of carbon another heat source is needed. X-ray bursters and Superbursters Rossi Explorer satellite Energy output of a single burst equal our suns solar output for a decade! Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016 The crust of an accreting neutron star is a unique environment for nuclear reactions Atmosphere: Accreted H/He Ocean: heavy elements Crust Carbon + heavy elements Density depth ~10 5 g/cm 3 5 m ~10 9 g/cm 3 30 m ~10 10 g/cm m Outer crust of an accreting neutron star Structure of an accreting neutron star crust Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016 Fusion of Neutron-rich Light Nuclei One potential heat source, proposed (Horowitz et al.) to heat the crust of neutron stars and allow 12 C fusion, is the fusion of neutron-rich light nuclei (ex. 24 O + 24 O) -- More recently mid-mass nuclei have been suggested. 24 O + 24 O Fusion: If valence neutrons are loosely coupled to the core, then polarization can result and fusion enhancement will occur 24 O is currently inaccessible for reaction studies Instead study other neutron rich isotopes of oxygen (18,19,20, 21) O on 12 C ( 19,20 O are radioactive) 16 O Core valence neutrons Horowitz et al., Phys. Rev. C 77, (2008) Umar et al., Phys. Rev. C 85, (2012) Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016 R.T. deSouza, S. Hudan, V.E. Oberacker, S. Umar Phys. Rev. C (2013) Density constrained TDHF calculations Damped dipole oscillation and presence of surface waves clearly visible. Fusion events and deeply inelastic events dominate at these near barrier energies Fusion is distinguished from deeply inelastic collisions by the existence of a single heavy nucleus after the collision Quantum mechanical calculations can also be performed to investigate the fusion process Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016 1)Nuclear astrophysics: Nuclear reactions in outer crust of a neutron star 2) Nuclear physics: dynamics of fusing two neutron-rich nuclei Motivation 24 O + 24 O not possible Measure fusion in 16,18,19,20 O + 12 C 19 O and 20 O are radioactive beams! Challenge: Radioactive beams are/will be available at intensities of ~10 3 10 5 ions/sec a million times less intensity than previously used in fusion studies. Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016 When comparing 18 O + 12 C to 16 O + 12 C DC-TDHF predicts a larger increase as compared to the experimental data. What happens (in reality) to the fusion cross-section as the oxygen nucleus becomes increasingly neutron-rich? Systematic fusion data to address this question is necessary! Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016 What do we want to measure? Excited nucleus decays: To measure the fusion cross-section we need to count the number of evaporation residues relative to the number of incident O nuclei Emission of evaporated particles kicks evaporation residues off of zero degrees Target Beam Evap. Residue Evaporation residuesEvaporated particles Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016 A simple counting experiment Measure the number of beam particles by counting them individually Count the number of residues Reciprocal of target thickness Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016 Method for Identifying Evaporation Residues Stop time Energy Start time Beam Residue To distinguish fusion residues from beam particles, one needs to measure: Energy of the particle Time of flight of the particle 18 O beam was provided by the Tandem van de Graaf accelerator at Florida State University (Feb. 2014) 18 E lab = 16 36 MeV I Beam ~ x10 5 p/s Wiedenhover et al., (5 th Int. Conf. on Fission & Prop. of Neutron-rich Nuclei, 2012)Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016 Gridless MCP Detector Minimize extraneous material in the beam path Crossed electric and magnetic field transports electrons from secondary emission foil to the microchannel plate (MCP) 20 neodymium permanent magnets produce magnetic field (~85 gauss) 6 grid plates produce electric field (~101,000 V/m) C foil frame biased to V MCP with 18 mm diameter Time resolution (MCP-MCP) ~ 350 ps Beam B E Bowman et al., Nucl. Inst. and Meth. 148, 503 (1978) Steinbach et al., Nucl. Inst. and Meth. A 743, 5 (2014) Si Detector New design (S5) from Micron Semiconductor Single crystal of n-type Si ~ 300 m thick Detector acts as a reverse biased diode Segmented to provide angular resolution Used to give both energy and time information S5 (T2) Si Design Pies16 Rings 6 24 ring segments Inter-strip width50 m Entrance widow thickness mSteinbach et al., Nucl. Inst. and Meth. A 743, 5 (2014) deSouza et. al., Nucl. Inst. and Meth. A 632, 133 (2011) Fast timing electronics gives timing resolution of ~ 450 ps (Need ~ 1 ns time resolution) Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016 18 O + 12 C Measurement at Florida State U. Time-of-flight of beam measured between US and Tgt gridless MCP detector Elastically scattered beam particles and evaporation residues: Time of flight measured between Tgt MCP and Si detectors Energy measured in annular Si detectors (T2, T3) 18 O BEAM US MCP Detector Tgt MCP Detector T2 ~ 130 cm ~ 13 cm PMT T3 LCP Det. Array 7 CsI(Tl)/photodiode detectors used to measure light charged particles PMT (coupled to plastic scintillator) measures zero degree beam particles Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016 Identifying Evaporation Residues 18 O + 12 E Lab = 36 MeV Rudolph et al., Phys. Rev. C 85, (2012) Rudolph, Masters Thesis, IU, 2012 Evaporation residues are clearly identified from Elastic scattering and beam scatter particles Alpha particles are also clearly distinguished Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016 18 O + 12 C Fusion Excitation Function Measured the cross section for E CM ~ 5.3 14 MeV matches existing data Extends cross-section measurement down to ~800 b level (~30 times lower than previously measured) Fit experimental data with penetration of an inverted parabolic barrier (Wong formalism) 18 O + 12 C : Comparison with DC-TDHF Even with inclusion of pairing the theoretical excitation function has the wrong SHAPE! In the sub-barrier region DC-TDHF significantly under-predicts the cross-section Above the barrier the quantity expt. / DC-TDHF is roughly constant at 0.8 Below the barrier the quantity expt. / DC-TDHF increases from 0.8 to approximately 15. Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016 What can we learn from the emitted particles (protons, alphas, and neutrons)? The velocity (energy) distribution of the particles is described by a Maxwell- Boltzmann distribution If the gas is in equilibrium with the liquid then measuring the velocity (energy) distribution of gas particle teaches us about the temperature of the liquid As the nucleus can be viewed as a charged droplet, measuring the energies of the emitted particles can provide information on the temperature of the nuclear system. Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016 Yield of particles decreases as incident energy decreases. Distributions can be characterized by a first and second moment (average value and width). Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016 increases roughly linearly with increasing bombarding energy. () first increases linearly then appears to saturate with increasing bombarding energy. A statistical model (PACE4) does a reasonable job of describing the widths but under-predicts the average energy of the particles. Turning to the average energy and width of the distribution Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016 The yield of particles The experimental cross-section decreases as the bombarding energy decreases At E c.m. = 14 MeV, is close to fusion As the bombarding energy decreases, alpha emission becomes a smaller and smaller fraction of the fusion cross- section. A statistical model, evapOR, that describes the de-excitation of excited nuclei does a poor job of describing the dependence of on bombarding energy. Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016 Energy and angular distribution of ERs in 18 O + 12 C Energy and angular distributions of evaporation residues (ERs) reveal a clear component associated with alpha emission. This component is under- predicted by the statistical model codes PACE4 and evapOR. Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016 Decay channels following fusion The large angle part of the ER angular distribution is populated almost exclusively by and +n decay with the largest angles populated by single emission only. The small angle region of the distribution is dominated by nucleon emission, principally 2n and n+p In order to explain the observed angular distribution it is necessary to increase the single emission by a factor of 11 and the n+ by a factor of 1.9; Due to the increase in the emission channels, it is necessary to reduce the nucleon emission channels by to reproduce the small angle portion of the spectrum Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016 19 O produced by: 18 ~68 MeV Production target of D 2 gas 300 torr at 77K 19 O filtered from beam using RESOLUT facility Intensity of 19 O: 1-2x10 4 pps Beam tagging by E-TOF Target: 100 g/cm 2 carbon foil T2: Lab = ; T3: Lab = TOF between target-MCP and Si (T2, T3 ) Radioactive beams at Florida State : The RESOLUT facility Ion chamber Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016 Radioactive beams at Florida State : The RESOLUT facility 18 O O O 6+ 19 O clearly resolved from 18 O over 3m flight path by TOF Simultaneous measurement of 19 O + 12 C and 18 O + 12 C Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016 Identification of ERs for 19 O + 12 C Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016 Fusion enhancement for 19 O + 12 C The excitation function for 18 O matches the results of the high resolution measurement previously performed within the statistical uncertainties. At all energies 19 O + 12 C is associated with a significantly large cross-section. Wong formalism (parabolic barrier) 18 O 19 O Rc7.34 0.07 fm8.10 0.40 fm V7.62 0.04 MeV7.73 0.61 MeV h /2 2.86 0.09 MeV6.38 2.50 MeV Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016 Fusion enhancement for 19 O + 12 C Above the barrier, the fusion cross- section for 19 O is roughly 20% larger than that for 18 O Just above the barrier at ~9 MeV the fusion cross-section for 19 O increases dramatically as compared to 18 O. At the lowest energy measured the cross- section for 19 O exceeds that for 18 O by approximately a factor of three. Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016 Conclusions Developed an efficient method to measure the fusion excitation function for low intensity radioactive beams at energies near and below the barrier. For 18 O + 12 C: We have measured the fusion cross-section down to the 800 b level a factor of ~30 lower than previously measured. In the sub-barrier regime the cross-section is substantially larger than that predicted by the DC-TDHF model suggesting a narrower barrier. Alpha emission is substantially enhanced over the predictions of the statistical model codes. For 19 O + 12 C: First measurement of fusion in this system indicates a significant enhancement of fusion due to the presence of a single extra neutron as one approaches the barrier. Romualdo deSouza, Univ. of Kentucky, Feb. 4, 2016 Tracy Steinbach, Jon Schmidt, and Dr. Sylvie Hudan At Florida State University Justin Vadas Dr. V. Singh Blake WigginsRomualdo deSouza, Univ. of Kentucky, Feb. 4, 2016 Additional slides Is fusion of neutron-rich light nuclei enhanced relative to -stable nuclei? Density constrained TDHF calculations TDHF provides good foundation for describing large amplitude collective motion 3D cartesian lattice without symmetry restrictions Skyrme effective nucleon-nucleon interaction (SLy4) Ion-ion potential calculated as: V(R) = E DC (R) E A1 E A2 Comparison of emission in similar systems 100 feet Good cheese, cider, and an accelerator facility with excellent technical support ISOL technique Primary beam: Ar Production target: C(graphite) CIME reacceleration: ~10 MHz 20 O 2+ t 1/2 : 13 s